16 PHYSICS. 
of two inclined planes, the height of each being half the thickness of 
' the back of the wedge. When a wedge is driven into a 
piece of wood up to the head, the wood at each side is forced 
back as far as half the thickness of the head. This has 
been done gradually by the sides of the split being forced 
up the inclined planes -formed by the sides of the wedge. 
Now, as a certain force balances a greater resistance on an 
inclined plane the longer the plane is, it follows that the 
longer the wedge used the greater is the power gained by 
using it, whether in splitting an object, or in raising a weight. 
Ships are raised in docks by driving wedges under the 
keel. Cutting and piercing instruments, as the plough, all — 
act on the principle of the wedge. A familiar illustration of the prin- 
ciple is seen ‘in the case of one glass tumbler placed within another, very 
little pressure on the uppermost one being sufficient to burst the lower. 

HYDROSTATICS. 
We have seen (COHESION, p. 3) that all matter exists in one or other 
of the three states, solid, liquid, and gaseous. Liquids and,gases have a 
general resemblance to each other in this respect, that their particles seem 
at liberty to glide about among one another without friction: they flow, 
and have hence received the common name of fluids, from Latin jluo, to 
flow. All liquids and gases have a certain degree of fluidity ; but the 
property which chiefly distinguishes them is elasticity. A quantity of gas 
may be compressed into much less than its ordinary bulk, and when the 
pressure is removed it will return to its original volume; but no ordinary 
pressure produces any sensible compression on water or any other liquid. 
Liquids are thus practically incompressible, and therefore practically 
inelastic. The phenomena of liquids are of two kinds, corresponding to 
those of solids—the phenomena of liquids at rest, or in equilibrium, and 
the phenomena of liquids in motion. The department of Physics which 
treats of liquids at rest is called Hyprostatics, from Greek hydor, water, 
and statiké, at rest. .A few of the facts or laws in connection with this 
subject are now to be considered. 
1, The fundamental principle of hydrostatics is, that when pressure 
as exerted on any part of the surface of a liquid, that pressure is trans- 
mitted to all parts of the liquid, and is exerted equally in all directions. 
That it must be so is evident from the nature of a fluid, whose 
particles are perfectly movable among one another, so that any particle 
could never be at rest unless when equally pressed on all sides. The 
first inference from this is, that the pressure of a liquid on any surface is 
